Thyroid transcription factor 1 (TTF-1) positivity is typically associated with NSCLC [72], however, several studies have shown that TTF-1 staining in the metastatic setting does not confirm lung origin [73, 74], and in fact, ~20–40% of poorly-differentiated lung adenocarcinomas are TTF-1 negative [75]. This evidence concerns the gene TTF1 and lung adenocarcinoma.